Metabolic Changes with Base-Loading in CKD

Metabolic acidosis in chronic kidney disease: mere consequence or also culprit?

Metabolic acidosis is a frequent complication in non-transplant chronic kidney disease (CKD) and after kidney transplantation. It occurs when net endogenous acid production exceeds net acid excretion. While nephron loss with reduced ammoniagenesis is the main cause of acid retention in non-transplant CKD patients, additional pathophysiological mechanisms are likely inflicted in kidney transplant recipients. Functional tubular damage by calcineurin inhibitors seems to play a key role causing renal tubular acidosis. Notably, experimental and clinical studies over the past decades have provided evidence that metabolic acidosis may not only be a consequence of CKD but also a driver of disease. In metabolic acidosis, activation of hormonal systems and the complement system resulting in fibrosis have been described. Further studies of changes in renal metabolism will likely contribute to a deeper understanding of the pathophysiology of metabolic acidosis in CKD. While alkali supplementation in case of reduced serum bicarbonate < 22 mmol/l has been endorsed by CKD guidelines for many years to slow renal functional decline, among other considerations, beneficial effects and thresholds for treatment have lately been under intense debate. This review article discusses this topic in light of the most recent results of trials assessing the efficacy of dietary and pharmacological interventions in CKD and kidney transplant patients.

Urine tricarboxylic acid cycle signatures of early-stage diabetic kidney disease

INTRODUCTION

Urine tricarboxylic acid (TCA) cycle organic anions (OAs) are elevated in diabetes and may be biomarkers for diabetic kidney disease (DKD) progression.

OBJECTIVES

We assessed associations of 10 urine TCA cycle OAs with estimated glomerular filtration rate (eGFR) and eGFR slope.

METHODS

This study is ancillary to the Simultaneous Risk Factor Control Using Telehealth to SlOw Progression of Diabetic Kidney Disease (STOP-DKD) Trial-a randomized trial of pharmacist-led medication and behavior management in 281 patients with early to moderate DKD at Duke from 2014 to 2015. We used linear mixed models to assess associations of urine TCA cycle OAs with outcomes and modelled TCA cycle OAs as: (1) the average of z-scores for each OA; and (2) principal component (PC) scores derived by principal component analysis (PCA). Untargeted urine metabolomics were added for additional discovery.

RESULTS

Among 132 participants with 24 h urine samples (50% men; 58% Black; mean age 64 years [SD 9]; mean eGFR 74 ml/min/1.73m2 [SD 21] and median urine albumin-to-creatinine [UACR] 20 mg/g [IQR 8-95]), PCA identified 3 OA metabolite PCs. Malate, fumarate, pyruvate, α-ketoglutarate, lactate, succinate and citrate/isocitrate loaded positively on PC1; methylsuccinate, ethylmalonate and succinate loaded positively on PC2; and methylmalonate, ethylmalonate and citrate/isocitrate loaded negatively on PC3. Over a median follow-up of 1.8 years (IQR, 1.2 to 2.2), higher average OA z-score was strongly associated with higher eGFR after covariate adjustment (p = 0.01), but not with eGFR slope (p = 0.9). Higher PC3, but not other PCs, was associated with lower eGFR (p < 0.001). Conditional random forests and smooth clipped absolute deviation models confirmed methylmalonate, citrate/isocitrate, and ethylmalonate, and added lactate as top ranked metabolites in models of baseline eGFR (R-squared 0.32 and 0.33, respectively). Untargeted urine metabolites confirmed association of urine TCA cycle OAs with kidney function.

CONCLUSION

Thus, lower urine TCA cycle OAs, most notably lower methylmalonate, ethylmalonate and citrate/isocitrate, are potential indicators of kidney impairment in early stage DKD.

Effect of Bicarbonate on Net Acid Excretion, Blood Pressure, and Metabolism in Patients With and Without CKD: The Acid Base Compensation in CKD Study

RATIONALE & OBJECTIVE

Patients with CKD are at elevated risk of metabolic acidosis due to impaired net acid excretion (NAE). Identifying early markers of acidosis may guide prevention in chronic kidney disease (CKD). This study compared NAE in participants with and without CKD, as well as the NAE, blood pressure (BP), and metabolomic response to bicarbonate supplementation.

STUDY DESIGN

Randomized order, cross-over study with controlled feeding.

SETTING & PARTICIPANTS

Participants consisted of 8 patients with CKD (estimated glomerular filtration rate 30-59mL/min/1.73m² or 60-70mL/min/1.73m² with albuminuria) and 6 patients without CKD. All participants had baseline serum bicarbonate concentrations between 20 and 28 mEq/L; they did not have diabetes mellitus and did not use alkali supplements at baseline.

INTERVENTION

Participants were fed a fixed-acid-load diet with bicarbonate supplementation (7 days) and with sodium chloride control (7 days) in a randomized order, cross-over fashion.

OUTCOMES

Urine NAE, 24-hour ambulatory BP, and 24-hour urine and plasma metabolomic profiles were measured after each period.

RESULTS

During the control period, mean NAE was 28.3±10.2 mEq/d overall without differences across groups (P=0.5). Urine pH, ammonium, and citrate were significantly lower in CKD than in non-CKD (P<0.05 for each). Bicarbonate supplementation reduced NAE and urine ammonium in the CKD group, increased urine pH in both groups (but more in patients with CKD than in those without), and increased; urine citrate in the CKD group (P< 0.2 for interaction for each). Metabolomic analysis revealed several urine organic anions were increased with bicarbonate in CKD, including 3-indoleacetate, citrate/isocitrate, and glutarate. BP was not significantly changed.

LIMITATIONS

Small sample size and short feeding duration.

CONCLUSIONS

Compared to patients without CKD, those with CKD had lower acid excretion in the form of ammonium but also lower base excretion such as citrate and other organic anions, a potential compensation to preserve acid-base homeostasis. In CKD, acid excretion decreased further, but base excretion (eg, citrate) increased in response to alkali. Urine citrate should be evaluated as an early and responsive marker of impaired acid-base homeostasis.

FUNDING

National Institute of Diabetes and Digestive and Kidney Diseases and the Duke O'Brien Center for Kidney Research.

TRIAL REGISTRATION

Registered at ClinicalTrials.gov with study number NCT02427594.

Veverimer: An Emerging Potential Treatment Option for Managing the Metabolic Acidosis of CKD

Sodium bicarbonate is the mainstay treatment of the metabolic acidosis of chronic kidney disease but associated concerns center on administering sodium to patients with hypertension and sodium-retentive states. Veverimer (formerly referred to as TRC101), a drug candidate for which Tricida, Inc is seeking approval from the US Food and Drug Administration, is a novel nonabsorbable polymer that binds hydrogen cations and chloride anions in the gastrointestinal tract and then is excreted fecally, thereby increasing serum bicarbonate concentration without administering sodium. We examine the published evidence on the investigational use of veverimer in patients with chronic kidney disease and metabolic acidosis. We highlight the achieved increase in serum bicarbonate concentration without coadministering sodium, effects on physical functioning, and the safety record of the drug. We also scrutinize certain unanticipated findings: a lack of dose dependency in the increase in serum bicarbonate concentration observed and that despite the presumed large hydrogen chloride losses in feces, veverimer induces an isochloremic increase in serum bicarbonate concentration that is accompanied by a decrease in serum anion gap. We propose likely explanations for these puzzling findings and raise questions about veverimer's mode of action and its potential interaction with colonic bacterial flora. Additional work is required to fill these knowledge gaps that could have important clinical implications.

Spot urinary citrate-to-creatinine ratio is a marker for acid-base status in chronic kidney disease

Due to multiple compensating mechanisms, the serum bicarbonate concentration is a relatively insensitive marker of acid-base status; especially in chronic kidney disease (CKD). This is a major drawback that impairs the ability to diagnose acid excess or monitor alkali therapy. We postulated that it is more logical to measure the compensatory defense mechanism(s) rather than the defended parameter, which remains normal if the compensation is successful. Therefore, a retrospective cross-sectional study was performed in 1733 stone formers along with a prospective cross-sectional study of 22 individuals with normal kidney function and 50 patients in different stages of CKD. While serum bicarbonate was flat and did not fall below the reference range until near CKD stage 5, citrate excretion (24-hour urinary citrate excretion rate; urinary citrate-to-creatinine ratio, in the retrospective analysis, and spot urinary citrate-to-creatinine ratio in the prospective study) progressively and significantly declined starting from CKD stage 2. Following an acute acid load in 25 participants with a wide range of estimated glomerular filtration rates, the urinary citrate-to-creatinine ratio inversely and significantly associated with acid accumulation, whereas serum bicarbonate did not. We compared changes in serum bicarbonate and urinary citrate-to-creatinine ratio in response to alkali therapy in patients with CKD stage 3 or 4 started on potassium citrate in our kidney stone database. With alkali therapy, there was no change in serum bicarbonate, but the urinary citrate-to-creatinine ratio rose consistently in all patients adherent to potassium citrate therapy. Thus, the urinary citrate-to-creatinine ratio (the defense mechanism) is a potential easily implementable, pragmatic, and a superior parameter to serum bicarbonate (the defended entity) to assess acid-base status, and monitor alkali therapy. Additional studies are needed before a clinical test can be devised.

APOL1, Acid Load, and CKD Progression

INTRODUCTION

High dietary acid load and metabolic acidosis are associated with an accelerated decline in kidney function and may contribute to the observed heterogeneity in end-stage renal disease (ESRD) risk according to APOL1 genotype. Our objective was to examine the associations of metabolic acidosis and dietary acid load with kidney disease progression, according to APOL1 genotype, among individuals with chronic kidney disease (CKD).

METHODS

We studied 1048 African American participants in the Chronic Renal Insufficiency Cohort. Metabolic acidosis was defined as blood levels of serum bicarbonate less than 22 mEq/L, and dietary acid load was quantified by potential renal acid load (PRAL) using data from the Diet Health Questionnaire. APOL1 status was defined as having 2 risk variants, consisting of either possible combination of the G1 and G2 risk alleles. We tested associations of APOL1 and dietary and metabolic acidosis with CKD progression, defined as time to ESRD or 50% decline in eGFR.

RESULTS

During a median follow-up period of 7 years, 379 participants had an incident CKD progression event (6.4 events per 100 person-years). After full adjustment, among participants with 2 APOL1 variants, the analysis failed to detect an association between metabolic acidosis or dietary acid load and CKD progression (hazard ratio [HR], 1.03; 95% confidence interval [CI], 0.96-1.11 per 1 mEq/L higher serum bicarbonate and an HR, 1.03; 95% CI, 0.92-1.15 per 10 mEq/L higher PRAL). Similar associations were noted among participants without the APOL1 high-risk genotype.

CONCLUSION

In a population at high risk of developing ESRD, metabolic acidosis and dietary acid load were not associated with CKD progression.